> Fit Guidelines> Measurement
BikeDynamics - Bike Fitting Specialists
Before describing the technologies used for measurement, it is important to discuss
why and what we are trying to measure. The data taken during the bike fitting process
falls into two categories:-
The performance monitoring is via the turbo trainer and optional heart rate monitor. We need to ensure that suitable effort levels are expended during the fitting process and that they are repeatable between measurements. Using a Computrainer Pro Ergometer we are able to continually monitor power, roadspeed, cadence, heartrate and the difference in contribution between left and right legs.
The anatomical measurements are the heart of the professional fitting process and are entirely dependent upon correct identification of body geometry reference points. The three most important locations are the hip, knee and ankle joints.
The ankle is the easiest to pinpoint by virtue of the lateral malleolus (outer bony lump) being quite prominent. The hip joint is trickier and is defined by the top of the greater trochanter on the femur. Prod hard enough and you should be able to feel this. The knee joint needs a little care as there is no obvious bony protrusion in the right place and the instantaneous centre of the joint moves as the knee flexes! The position we choose is the gap between femur and tibia, centrally within the joint.
Static Measurement Tools.The classic bike fitting tools for static measurement are a tape measure, plumb bob, and a goniometer for measuring joint angles. We also use an inclinometer and laser to check alignments.
We also have a purpose-designed cleat jig to ensure correct fore-aft and angular positioning.
Footdisc.The Footdisc is a dry replacement for the traditional wet feet test. Thermo-dynamic crystals in the device change colour when exposed to body heat, clearly indicating if the foot is high arched, neutral or flat.
Fore-foot Measurement Device.The fore-foot measurment device defines the extent of either a varus or valgus tilt of the fore-foot. Used in conjunction with knee tracking in the frontal plane, this help to define the position and number of wedges or shims in the shoes.
For optimum results, bike fitting is best done dynamically i.e. at the loads the
cyclist applies to the bike whilst riding. To enable this we need to be able to
accurately capture and define the cycling motion. BikeDynamics uses three dynamic measurement tools:-
Look Keo Fit.The Look Keo Fit system enables the perfect rotational alignment of your cleats and selection of appropriate float. The electronic pedal senses the position of a magnet placed in each cleat over a 3-4 minute pedalling cycle. The system then displays the actual foot position relative to the cleat and the amount of float required. Most people use up to approx. 2 degrees of float, in which case the grey cleats are appropriate. More than 3.5 degrees of float calls for the red cleat whereas a black cleat could be considered when less than 0.5 degrees. An alignment tool is used to reposition the cleat to the required nominal position.
Dartfish Video Analysis.BikeDynamics uses Dartfish Pro Suite video analysis software in conjunction with two Canon HV30 HD cameras and a PC with a very fast processor. This configuration allows simultaneous recording from two cameras placed to give a 3m side and a 2.25m front view. Using HD 50i frame rates, fast shutter speeds and a well lit south facing room, sharp images are easily achievable, even with pedalling cadences of over 100rpm.
Examples of typical analysis techniques are as follows.
Side View - Video Analysis.Even before any analysis, just being able to see the rider either at normal speed, slow motion, or frame by frame is very powerful. We use the stability of the shoulder marker as one criteria for establishing a good fit. The movement of the shoulder, usually either a fore-aft 'kick' or vertical 'bob' can be very informative on rider flexibility and optimum position.
Side View - Joint Angles.The video analysis capability enables the key metrics of knee, hip, torso, ankle and shoulder angles to be measured at each key point around the pedal stroke. Guideline 'ranges' exist for these metrics, but the ideal position within the range will often be dictated by rider flexibility. For instance, the maximum knee angle range is usually 142.5-147.5 degrees, but people with very tight hamstrings will often not get above 140 degrees before becoming unstable. Very flexible people, usually women, will often comfortably tolerate knee angles up to 150 degrees.
Automatic Tracking.The ability for the analysis software to 'lock on' to a marker and follow it provides an excellent tool to monitor knee motion in the front plane. The ideal knee trajectory is a straight up and down vertical action with no lateral wobble. Unfortunately, very few people display this ideal, with the travel shown here being more typical. We use symmetry, consistency from stroke to stroke, difference between up and down strokes, hip, knee and foot alignment and vertical travel as criteria. Countermeasures to improve knee tracking can include cleat adjustment, pedal spacers, shims or wedges and increased arch support.
Computrainer ErgometerThe Computrainer PRO Ergometer offers the usual capability of monitoring power, cadence, speed and heart rate, but also provides the unique opportunity to measure torque at the rearwheel for every 15° of the pedal stroke. This can be presented as either a bar graph, or more usefully the polar plot shown here. This Spinscan analysis gives a very useful visual representation of the pedal stroke, whilst generating metrics such as 'Pedalling Efficiency', 'Average Torque Angles' and Left / Right balance.
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